Phase correction for frequency response function measurements

In modal testing, an impulse is often used to excite the structure and a linear transducer is used to measure the response. For these impact tests, two signals are measured: the impulsive force and the vibration response. Any lack of synchronization in the time domain acquisition of the two signals results in a frequency-dependent phase error in the frequency response function, or FRF. However, knowledge of the time delay may be used to correct the corresponding phase error. In this research, tests were conducted to measure the frequency-dependent phase error for a capacitive sensor and a frequency domain technique is proposed to correct the FRF. The method was validated using an FRF measurement of a cylindrical artifact mounted in a milling machine spindle.     

A novel neural network method for wind speed forecasting using exogenous measurements from agriculture stations

This paper proposes a novel ANN-based wind speed forecasting method based in the introduction of low-quality measurements as exogenous information, processed by six prediction models to perform one-hour-ahead enhanced forecasting. The models evaluated are classified in two groups: first, persistence and ARIMA, which are used as references, and secondly, the remaining four, based on neural networks. Model comparison is realized by applying two procedures. On the one hand, four quality indexes are assessed (the Pearson’s correlation coefficient, the index of agreement, the mean absolute error and the mean squared error), and the other hand, an ANOVA test and multiple comparison procedure are conducted. A backpropagation network with nine neurons in the hidden layer obtains improvements couples (mean absolute – mean squared error) of 23.92–47.48%, and 23.19–45.54% for the persistence and the ARIMA models, respectively. The paper provides strong practical evidence that traditional agricultural measurements are potentially capable of improving estimates and forecasts.     

塔式起重机动态分析的计算模型

分别按集中参数法及有限元法建立了塔式起重机结构系统的动力学模型．按集中参数法建立的少自由度模型包括四自由系统、三自由度系统、二自由度系统及单自由度系统，时二自由度系统及单自由度系统的结构固有频率给出了简化计算公式。通过实例验算证明，少自由度系统或简化计算公式的计算精度可以满足工程设计的要求。     

一种处理系统误差的新方法

测量数据处理中影响观测值的因素有2个部分：线性部分和某种干扰因素，干扰因素同观测量的关系是完全未知的，没有理由将其归入误差项，可以将其看成半参数模型中的非参数分量，即用非参数分量表达参数模型不完善的部分。本文首先利用自然样条函数法，找到符合条件的非参数自然插值样条函数；其次利用偏残差法并综合最小二乘法，导出了这种平差方法的解算公式；最后，用一个算例说明了此方法的优越性。     

Precision of stereological planar area predictors

Total planar area can be estimated based on sampling by a lattice of figures (e.g. point patterns, line segments, quadrats). General formulae are provided for the approximation of mean squared errors. The approximation formulae are products of the boundary length and of a parameter that depends only on the sampling scheme. An R package is provided by the authors for the numerical computation of the mean squared error formulae. The speed of convergence of the mean squared error approximation is assessed on the basis of several simulations. Several sampling schemes are compared in view of the approximated mean squared errors.     

一种频响函数灵敏度分析方法

频响函数灵敏度分析在结构动力优人和振动控制等许多领域中着广泛的应用,基于模态展开和幂级数展开原理,提出了一种频响函数灵敏度分析的模态展开法,为了减少模态展开时的模态截断误差,文中同时给出了一种模态加速了方法,该方法可大大提高模态截断时的灵敏度分析精度,通过对一数值害例计算表明,文中提出的方法非常有效。     

Multi-loop nonlinear control design for performance improvement of LTI systems

This paper puts forward a multi-loop nonlinear control (MLNC) strategy to overcome the limited performance of LTI controllers due to the so-called “waterbed” effect. According to “Bode's sensitivity integral”, increasing the bandwidth or additional integral gain of LTI controller to improve the low-frequency disturbance attenuation irrefutably increases the sensitivity to high-frequency disturbances or measurement noise. Hence, it is impossible to attain the best of both worlds in the case of linear controllers. Therefore, with an aim to improve the transient and steady state performance of linear controllers, in this paper, a nonlinear control framework using circle criterion method and saturation nonlinearity, which adjusts the integral gain based on the error threshold, is discussed. The global asymptotic stability (GAS) of the MLNC strategy is theoretically proved using LaSalle's invariance principle and experimentally validated using measured frequency response function (FRF). Moreover, the performance of the MLNC strategy is compared with that of the multi-loop linear control (MLLC) strategy on a benchmark magnetic levitation system for tracking application. The cumulative power spectral density (CPSD) of tracking error, which is used as the performance index to assess the overall closed loop performance, accentuates that MLNC can yield better steady state and transient performance compared to MLLC scheme.     

一种改进的基于响应耦合子结构法的刀尖点频响函数预测方法

针对现有的基于响应耦合子结构法(RCSA)的刀尖点频响函数预测方法需要辨识主轴-刀柄、刀柄-刀具结合面参数以及需要自制刀柄模型等引起的预测误差和预测过程复杂等问题,提出一种改进的基于RCSA的铣刀刀尖点频响函数预测方法。该方法首先改进已有的子结构划分方法,将机床-主轴-刀柄-刀具系统划分为机床-主轴-刀柄-部分刀杆、剩余刀杆和刀齿三个子结构;然后改进主轴-刀柄处转动频响函数的计算方法,通过铣刀的模态锤击实验采用反向RCSA和有限差分法计算机床-主轴-刀柄-部分刀杆结构的转动频响函数,并基于Euler梁模型计算出剩余刀杆、刀齿子结构的频响函数;最后将三个子结构的频响函数耦合确定刀尖点的预测频响函数。以一立式加工中心为研究对象,应用所提出的方法对铣刀刀尖点的频响函数进行了预测,并与其实测频响函数进行对比。对比结果表明：刀尖点的预测频响函数与实测频响函数符合程度较高,其预测、实测前三阶固有频率之间的误差在6.9%以内,所提出的方法可行有效、简单方便,且可直接基于铣刀的模态实验计算主轴-刀柄的频响函数,避免了相关结合面参数的辨识和刀柄模型的制作。     

APPLICATION OF FRF ESTIMATOR BASED ON ERRORS.IN-VARIABLES MODEL IN MULTI-INPUT MULTI-OUTPUT VIBRATION CONTROL SYSTEM

The FRF estimator based on the errors-in-variables(EV)model of multi-input multi-output (MIMO)system is presented to reduce the bias error of FRF H1 estimator.The FRF H1 estimator is influenced by the noises in the inputs of the system and generates an under-estimation of the true FRF. The FRF estimator based on the EV model takes into account the errors in both the inputs and outputs of the system and would lead to more accurate FRF estimation.The FRF estimator based on the EV model is applied to the waveform replication on the 6-DOF (degree-of-freedom) hydraulic vibration table.The result shows that it is favorable to improve the control precision of the MIMO vibration control system.     

Performance evaluation of two different neural network and particle swarm optimization methods for prediction of discharge capacity of modified triangular side weirs

Technical design of side weirs needs high accuracy in predicting discharge coefficient. In this study, discharge coefficient prediction performance of multi-layer perceptron neural network (MLPNN) and radial basis neural network (RBNN) were compared with linear and nonlinear particle swarm optimization (PSO) based equations. Performance evaluation of the model was done by using root mean squared error (RMSE), coefficient of determination (R²), mean absolute error (MAE), average absolute deviation (δ) and mean absolute relative error (MARE). Comparison of the results showed that both neural networks and PSO based equations could determine discharge coefficient of modified triangular side weirs with high accuracy. The RBNN with RMSE of 0.037 in test data was found to be better than MLPNN with RMSE of 0.044 and multiple linear and nonlinear PSO based equations (ML-PSO and MNL-PSO) with RMSE of 0.043 and 0.041, respectively. However, due to their simplicity, PSO based equations can be sufficient for use in practical cases.     

电火花加工8418钢的工艺预测模型

在电火花加工中,加工工艺指标的结果与工艺参数的设置密切相关。一般情况下,操作者在进行实际执行之前,只能根据以往的加工规律以及经验手段对其结果进行预判,达到预先评估加工结果的目的。针对这一情况,提出一种适用于电火花加工工艺指标结果预测的模型,该模型的建立是基于支持向量回归理论的数学方法,并利用遗传算法优化该方法中的各参数。以电火花加工8418模具钢为例,结合正交试验方法和经验加工方法选取加工工艺参数,并记录工艺指标结果。为保证EDM工艺指标预测模型的准确性,将试验数据随机分成训练集和测试集,利用训练集训练EDM工艺指标预测模型,可得加工时间模型均方误差T_(MSE)=0.95′10~(-4),平方相关系数T_(R2)=0.99 1;工件去除率模型均方误差MRR_(MSE)=1.02′10~(-4),平方相关系数MRR_(R2)=0.999 3;电极损耗率模型均方误差EWR_(MSE)=1.11′10~(-4),平方相关系数EWR_(R2)=0.998 9。再利用测试集验证该模型,可见预测结果与试验结果之间的误差在5%以内,从而证明电火花加工8418钢工艺预测模型的准确性和有效性。     

基于参数模型的频响函数估计方法

针对传统的非参数频响函数估计方法不适用于短记录数据的情况,提出了一种基于公分母模型的参数化频响函数估计方法.该方法使用输入输出信号的快速傅里叶变换(输入输出谱)作为主要数据,通过最小二乘法求解参数,利用奇异熵技术对系统进行定阶.最后采用一个二层框架的仿真算例对所提出的算法进行了验证.仿真算例结果表明,通过响应信号的奇异...     

Response selection and dynamic damper application to improve the identification of multiple input forces of narrow frequency band

In identifying input forces by using FRF matrices and response measurements, improper selection of the response measurement position and the excitation frequency, if it coincides with the natural frequencies of the structure, may cause doubtful accuracy and effectiveness. This paper presents two methods to improve force identification. The first is by improving the conditioning of the system FRF matrix by a proper selection of the measurement positions. An expression for the covariance matrix representing the uncertainty on the force estimate is defined. Then the response selection algorithm, which is both systematic and effective, is derived. The basic strategy in selecting the response positions is to let the smallest singular value be as large as possible. The second method is to modify the structure by attaching a dynamic damper at a suitable location to minimise the ill-conditional nature of the FRF matrix especially near a resonance frequency. The methods suggested are tested numerically and experimentally in two examples: a beam structure and a rectangular plate structure. The test results show that the response selection and the addition of a dynamic damper are very effective in solving ill-conditioning problems, especially when the structure has a large degree of freedom and when the excitation frequencies are near one of the structure's natural frequencies.     

Hybrid time–frequency methods for non-stationary mechanical signal analysis

This paper presents a family of hybrid time–frequency methods to be used as a tool for transient signal analysis. These methods are based on autoregressive models of the signal, and on the maximum likelihood method. A classical Wigner–Ville distribution and a parametric time–frequency methods are used for comparison purposes. The characteristics, advantages and disadvantages, of non-parametric, parametric and hybrid methods are discussed and their performances are compared by analysing actual data. The superiority of time–frequency resolution of the hybrid methods is pointed out over that of the parametric and non-parametric ones. Three practical examples of transient signals illustrate the discussion: the vibration signal issued from a damaged gearbox, a signal from a pendular scratch test presenting beating phenomenon and the vibration signal measured in an electrical motor during start-up.     

基于相对误差平方和的神经网络预测镁合金多轴疲劳寿命

提出一种以相对误差平方和(Surm squared relative error,SSRE)作为误差性能函数的反向传播(Back propagation,BP)神经网络算法(SSRE-BP),针对3种不同镁合金AZ31B、ZK60和AZ61A在单轴拉压、纯扭、45°比例和90°圆形非比例等4种不同加载路径下的疲劳寿命进行预测。并与以均方误差(Mean squared error,MSE)作为误差性能函数的传统BP神经网络(MSE-BP)以及基于临界平面法的SWT疲劳损伤模型预测的结果进行比较。结果表明,在3种镁合金材料总共138组疲劳数据中,神经网络只有一组预测值在3倍偏差界限外,而用SWT预测结果分别有16组、13组、10组数据在3倍偏差界限外。两种BP神经网络能够较好地预测镁合金不同加载路径下的疲劳寿命,相比于SWT疲劳模型预测的寿命在精度上有较大幅度的提升。其中,SSRE-BP算法的精度略高于传统的MSE-BP算法。     

刚体惯性参数的动态测试研究

根据刚体动力学原理、频响函数曲线的质量线理论,推导出表征刚体惯性参数的基本原理。然后提出频响函数的拟合方法,并通过误差分析,研究和改进试验方法及优化数据处理,对某圆柱形质量块惯性参数的测试值与理论值比较,标定整个测量系统的误差。最后以某型汽车动力总成为研究对象,基于试验模态分析技术完成该动力总成相关惯性参数的动态测试。     

Impact of forecasting methods on variance ratio in order-up-to level policy

This paper considers the impact of forecasting methods on the bullwhip effect for a simple replenishment system in which a first-order autoregressive process describes the customer demand and an order-up-to inventory policy characterizes the replenishment decision. The impact of exponential smoothing and minimum mean squared error forecasting is measured for both the bullwhip effect and inventory variances. Previous similar studies have focused on investigating the impact of forecasting methods on bullwhip effect. However, little research has been carried out to explore the impact of forecasting methods for both bullwhip effect and inventory variances. Through simulation experiments, it has been found that depending on the structure of the demand process, the appropriate selection of forecasting technique can reduce, or even eliminate (i.e., “dewhip”) the bullwhip effect. However, in terms of inventory variances it has been shown that the inventory variances for the exponential smoothing are greater than the minimum mean squared error forecasting method and that gap increases as lead time increases. These findings will help companies to choose the appropriate forecasting technique depending on the nature of demand. These guidelines can help companies to reduce the bullwhip effect and inventory variances across supply chain.     

冷水机组机架减振效果分析

根据冷水机组结构特性设计了一种减振机架,并利用ANSYS对其固有频率和模态进行了计算;提出了一种利用频响函数法对冷水机组机架的减振效果进行评估的方法,通过ANSYS仿真得到了其频响函数,最后通过实验验证了该方法的可行性。结果表明,该机架具有明显的减振效果,频响函数法可以有效地评估机组或部件的减振效果。     

Analysis of swept-sine runs during modal identification

Experimental modal analysis of large aerospace structures in Europe combine nowadays the benefits of the very reliable but time-consuming phase resonance method and the application of phase separation techniques evaluating frequency response functions (FRF). FRFs of a test structure can be determined by a variety of means. Applied excitation signal waveforms include harmonic signals like stepped-sine excitation, periodic signals like multi-sine excitation, transient signals like impulse and swept-sine excitation, and stochastic signals like random. The current article focuses on slow swept-sine excitation which is a good trade-off between magnitude of excitation level needed for large aircraft and testing time. However, recent ground vibration tests (GVTs) brought up that reliable modal data from swept-sine test runs depend on a proper data processing. The article elucidates the strategy of modal analysis based on swept-sine excitation. The standards for the application of slowly swept sinusoids defined by the international organisation for standardisation in ISO 7626 part 2 are critically reviewed. The theoretical background of swept-sine testing is expounded with particular emphasis to the transition through structural resonances. The effect of different standard procedures of data processing like tracking filter, fast Fourier transform (FFT), and data reduction via averaging are investigated with respect to their influence on the FRFs and modal parameters. Particular emphasis is given to FRF distortions evoked by unsuitable data processing. All data processing methods are investigated on a numerical example. Their practical usefulness is demonstrated on test data taken from a recent GVT on a large aircraft. The revision of ISO 7626 part 2 is suggested regarding the application of slow swept-sine excitation. Recommendations about the proper FRF estimation from slow swept-sine excitation are given in order to enable the optimisation on these applications for future modal survey tests of large aerospace structures.     

基于置信传播的多运动平台随机组网协同导航

在协同网络中,随着导航个体数目的增加,如何确定参与组网的导航平台并获取其状态与观测量的数据关联性是实现 小型规模的多运动平台协同导航的关键。 本文提出一种基于置信传播的多运动平台随机组网协同导航方法,采用随机有限集 对状态和量测量进行建模并附加标签,构造带有标签的多伯努利粒子滤波器,采用基于置信传播的多运动平台随机组网协同导 航方法,并利用运动目标平台与基站的绝对观测、运动目标平台与临近平台相对观测融合,通过粒子滤波器和置信传播对系统 中的量测信息进行处理,相比较于非参数置信传播算法扩展因子结构作为置信度算法的逼近,该算法实现概率数据关联进行导 航系统状态估计。 仿真和物理实验结果表明:非参数置信传播算法在不同基站和不同粒子数的解算结果性能较差;本文提出的 算法受基站个数和粒子个数的影响较小鲁棒性好,收敛性好,均方根误差不高于 0. 3 cm,精度高于前者一个数量级,该算法能 够有效获取导航平台的位置信息。